Realistic Interstellar Explorer Mission Concept Reaches 1000 AU
1. 00-0642-1
A Realistic Interstellar Explorer
Ralph L. McNutt, Jr., G. Bruce Andrews,
James V. McAdams, Robert E. Gold, Andrew G. Santo,
Douglas A. Ousler, Kenneth J. Heeres,
Martin E. Fraeman, and Bruce D. Williams
The Johns Hopkins University
Applied Physics Laboratory
Laurel, MD 20723-6099
June 6, 2000
NASA Institute for Advanced Concepts
2nd Annual Meeting
NASA Goddard Space Flight Center
Greenbelt, MD
2. 00-0642-2
Other Contributors
D. R. Haley, JHU/APL D. W. Dunham, JHU/APL
R. S. Bokulic, JHU/APL E. C. Roelof, JHU/APL
P. E. Panneton, JHU/APL R. E. Jenkins, JHU/APL
J. I. Von Mehlem, JHU/APL R. Westgate, JHU
L. E. Mosher, JHU/APL D. Lester, Thiokol Corp.
E. L. Reynolds, JHU/APL D. Read, Lockheed-Martin
R. W. Farquhar, JHU/APL D. Doughty, Sandia National
D. W. Sussman, JHU/APL Laboratories
Support from:
Tasks 7600-003 and 7600-039 from the NASA Institute for
Advanced Concepts (NIAC) under NASA Contract NAS5-98051
3. 00-0642-3
The Goals of Space Exploration Are at the
Boundaries of the Heliosphere and Beyond
Oort
Cloud
1
AU
4. 00-0642-4
Science Goals
Travel to the stars is the stuff that dreams are made of. There is
also a very scientifically compelling aspect as well. A mission
past the boundary of the heliosphere would yield a rich
scientific harvest.
• Explore the nature of the interstellar medium and its implications for
the origin and evolution of matter in the Galaxy.
• Explore the structure of the heliosphere and its interaction with the
interstellar medium.
• Explore fundamental astrophysical processes occurring in the
heliosphere and the interstellar medium.
• Determine fundamental properties of the universe, e.g., big-bang
nucleosynthesis, location of gamma-ray bursts (GRBs), gravitational
waves, and a non-zero cosmological constant.
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“Realistic” Propulsion Concepts Have
Intrinsically Large (100s of Tons) Dry Masses
Daedalus Fusion
Rocket (D-3He)
Sänger Photon Rocket
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Technology Extrapolations Sound
Too Good to Be True and May . . .
• Be driven by propulsion requirements
• Drive costs to scale of current GDP (or beyond!) if a
miracle does not occur
Bussard Ramjet NASA Breakthrough
Propulsion Physics Program
7. 00-0642-7
A Mission to the VLISM Is More Modest,
but Can Be Done in the Near Term
• The external
shock may be
~300 AU away
• So 1000 AU is
“clear”of the
influence of the
Sun on its
surroundings
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Mission Concept
• Reach a significant penetration into the Very Local Interstellar
Medium—out to ~1000 AU—within the working lifetime of the
probe developers (<50 years)
• To reach high escape speed, use a solar gravity assist (due to
Oberth, 1929):
(1) Launch to Jupiter and use a retrograde trajectory to eliminate
heliocentric angular momentum
(2) Fall into 4 solar radii from the center of the Sun at perihelion
(3) Use an advanced-propulsion system ΔV maneuver to increase
probe energy when its speed is highest to leverage rapid solar
system escape
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Enabling Technologies
• High Isp, high-thrust propulsion (for perihelion
maneuver, ~15 minutes)
• Carbon-carbon thermal shield
• Long-range, low-mass telecommunications
• Efficient Radioisotope Thermoelectric Generator (RTG)
• Low-temperature (<150K), long-lived (<50 yr) electronics
• <0.1 arc second pointing for data downlink
• Open loop control
• Fully autonomous operational capability with onboard
fault detection and correction
• Possible extension to multi-century flight times while
maintaining data taking and downlink operations
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Study Topics
• Architectures that allow launch on a Delta III-class vehicle
• Redundancies that extend probe lifetime to >1000 years;
software autonomy, safing
• Concept that links science, instruments, spacecraft
engineering, and reality
• 1000 AU, 50-year mission; extension to 1,000 years
(~20,000 AU)
• Optical downlink: data/attitude requirements
• Propulsion concepts: solar thermal, nuclear pulse, nuclear
thermal—search for higher speed
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Trajectory Toward
ε Eridani
• Launching toward a star
enables comparison of
local properties of the
interstellar medium with
integrated properties
determined by detailed
measurements of the
target-star spectrum.
• Additional planetary
flybys over-constrain
the trajectory design.
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Target Trajectory
• Target the Sun-similar star ε Eridani, a K2V dwarf main
sequence star 10.7 light years from Earth.
• A 2011 launch to Jupiter with a launch energy C3 = 117.1
km2/s2, and two years later, a 15.4 km/s perihelion burn near
the Sun sends the spacecraft at 20.2 AU/year.
• Launching toward a star enables comparison with locally
measured properties of the interstellar medium with integrated
properties determined by detailed measurements of the target-star
spectrum.
• This low-ecliptic latitude target minimizes the required
perihelion burn for a given asymptotic escape speed.
• The details of Jupiter’s orbit cause the launch energy
requirements to return to an optimum roughly once every 80
years for a launch toward ε Eridani.
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Solar System Escape Speed
• Varies as the inverse fourth root of the perihelion distance
• Varies as the square root of the perihelion ?V
vescape = (ΔV) 1
2 35.147
1
rp
4
• 1 AU/yr = 4.74 km/s
• A "now-technology" Interstellar Probe could supply a ?V of
1.56 km s–1 at perihelion, about one-tenth of what is
desirable. Perihelion distance = 3 RS => ~7.0 AU yr–1.
• To reach ~20 AU yr –1, the probe needs to be accelerated by
~10 to 15 km s –1 during about 15 minutes around perihelion
to minimize gravity losses.
14. 00-0642-14
Target Stars are Limited to Low Ecliptic
Declinations Unless Additional ΔV Is Provided
Star Distance
(Light Years)
Spectral
class
Habitable
planet
probability
Alpha Centauri A 4.3 G4 0.054
Alpha Centauri B 4.3 K1 0.057
Epsilon Eridani 10.8 K2 0.033
Tau Ceti 11.8 G8 0.036
70 Ophiuchi A 16.4 K1 0.057
Eta Cassiopeiae A 18.0 K9 0.057
Sigma Draconis 18.2 G9 0.036
36 Ophiuchi A 18.2 K2 0.023
36 Ophiuchi B 18.2 K1 0.020
Delta Pavonis 19.2 G7 0.057
82 Eridani 20.9 G5 0.057
Beta Hydri 21.3 G1 0.037
Data is from Dole [1964]
Designations A and B refer to components in multiple (bound) star
systems; the effect of multiple systems on the formation of stable
planetary orbits remains unknown
Star Spectral Class
a Right
Ascension
(deg)
d Declination
(deg)
Epsilon Eridani K2 48.29 -27.76
Tau Ceti G8 17.66 -24.77
Wolf 28 DG 13.28 0.19
Procyon F5 115.87 -16.00
191408 K3 297.06 -15.68
131977 A K5 228.26 -4.31
36 Ophiuchi K1 259.96 -3.54
16. 00-0642-16
Ulysses Stack Provided 15.4 km/s
• Chemical production of such high-speed changes in deep
space is not possible.
• Ulysses spacecraft and propulsion module were released
into Earth orbit from the Shuttle.
• Ulysses launch mass = 371 kg with 55 kg of science
payload.
• Total stack (mass 2-stage IUS + PAM-S upper stage) =
19.97 metric tons.
• 15.4 km/s over 5.8 minutes burn time BUT we need this
applied at 4 RS !
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Solar Thermal Propulsion Concept
• Being studied for high-Isp, low-thrust orbital
transfer vehicles
• Possible implementation for perihelion
maneuver with Isp ~1000 s at high thrust
18. 00-0642-18
Solar Thermal Propulsion
• Solar thermal propulsion uses energy from the Sun to heat a
low-molecular-weight working fluid to a high temperature
(~2400K) and expel the propellant mass from the system.
• An “obvious” choice for a working fluid is liquid hydrogen
(LH2); however, realistic cryostats carry a substantial mass
penalty for long-term LH2 storage for use as a working fuel at
perihelion.
• The use of ammonia enables standard pressurized titanium
tank technology, but dissociation of the ammonia requires
higher temperatures and so the specific impulse is lower.
• A bottleneck, in addition to mass, is providing sufficiently
rapid heat transfer to the fuel in the near-Sun environment.
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Nuclear Pulse Propulsion
Scalability to low-mass systems is problematic due to critical
mass of fission assemblies. Nuclear Thermal Propulsion may
be the real answer.
20. 00-0642-20
Nuclear Pulse Propulsion (“Orion”)
• Fission may provide the key element for the perihelion propulsion,
but only in a pulsed mode with low fission yields per pulse; we need
the fission energy of ~1.3 g of uranium–a total of about 13 tons of
TNT equivalent.
• The problem is the coupling of the momentum into the ship over
short time scales, ~10–8s. Transferring this impulse over such short
times typically causes stress to exceed the yield strengths of all
known materials.
• The Orion concept requires large masses for dealing with the
release of ~1 to 10 kT explosions; however, the spacecraft masses
tend to be large due to the power plant overhead.
• Ideally, a pulsed-mode autocatalytic reaction similar to the
operation of a pulse jet, e.g., the German V-1, is preferred. This type
of rocket would represent the next step past a gas core nuclear
engine.
22. 00-0642-22
Probe Configuration After Trans-Jupiter Injection
• The probe, thermal shield, and perihelion propulsion
stage are sized to fit within a Delta II shroud.
• The cocoon-like structure carries the probe past Jupiter
and into its perihelion maneuver at the Sun.
• The probe itself is small; most of the volume is occupied
by the thermal shield and perihelion propulsion system.
• Following the perihelion burn, the probe is mechanically
jettisoned from the cocoon structure to begin its 50-year
prime science mission.
24. 00-0642-24
Probe Architecture
• Following the perihelion burn, the probe mechanical
design consists of three main mechanical elements
–a Radioisotope Power Source (RPS) assembly, a
central support mast, and an optical dish.
• The RPS is placed at one end of the mast in order
to minimize the radiation dose to other spacecraft
components.
• At the other end of the mast is the large optical dish,
which faces away from the direction of travel and
back toward the solar system.
25. 00-0642-25
Mass and Power of Probe
Mass Power
Interstellar Probe (kg) (Watts)
Power System 10 —
Instruments 10 10
Structure 15 —
Communications 10 Intermittent,
uses energy
storage system
S/C Electronics 5 5
Totals 50 15
26. 00-0642-26
Probe Structural Configuration
• Instruments and spacecraft electronics boxes are placed
on the back of the optical dish and along the mast.
• Four booms, used for field measurements, are mounted
orthogonally to one another at the perimeter of the optical
dish.
• The spacecraft secondary battery and power control
system are placed inside of the mast roughly at its
midsection.
• The communication system laser is also placed inside the
mast and points out the end of the mast, through a small
hole in the 1-m optical dish, toward the hyperboloid
reflector.
28. 00-0642-28
Command and Data Handling
• The probe does not have a subsystem onboard that can be
strictly identified as a “command and data handling system.”
• Modules perform command and data handling functions but
are not aligned or connected to any particular subsystem or
task.
All processing is distributed and all tasks are only loosely
coupled with specific hardware.
• All the identical processor modules exceed 200 MIPS so that
any processor is capable of handling all the tasks onboard
single-handedly if necessary.
• Each module contains a central CPU, memory, and a wireless
communication submodule.
• If a processor fails, its task shifts to an unused processor. If no
spare processors are available, then some of the remaining
processors handle more than one task. In the worst case
scenario, when only one processor is still working, it alone
handles all tasks.
29. 00-0642-29
Mass and Power of Science Payload
Instrument Identifier Mass (kg) Power (W)
Plasma waves/dust detection PWD 1.5 2.5
Plasma/particles/cosmic rays
PPC 1.0 1.5
composition and spectra
Magnetometer (w/boom) MAG 3.0 0.5
Lyman- α imager LYA 1.0 2.0
Infrared imager IRI 1.5 1.5
Neutral atoms composition, density,
NAC 2.0 2.0
speed, temperature
Totals — 10.0 10.0
30. 00-0642-30
Prime Science Period
• Once two-way communication is lost, the Interstellar Probe
enters an autonomous mode with only infrequent downlinks.
• This is the prime science portion of the mission in which
(1) The probe maintains a slow spin with the spin axis
pointing back at the Sun. The spinning spacecraft
allows the science instruments to see the entire sky.
(2) The instruments collect and process their own data.
(3) At regular intervals, the probe points accurately toward
a Hubble-class receiving station, which is orbiting the
Earth, and then transmits the science data.
(4) Onboard processors continually monitor the health of
the probe and take corrective action(s) as required.
31. 00-0642-31
An RPS Provides Centuries of Power
for Very Long Missions
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Power System
• The power source for a deep-space interstellar
mission must be a Radioactive Power Source (RPS).
• For “closer” missions Pu-238 will suffice: one 15-W
Pu-238 ARPS satisfies steady-state power
requirements and weighs 4 kg.
• For “further” missions, Am-241 has a longer half-life,
but a lower operational temperature is problematic.
• Communication and attitude systems operate
intermittently and require significant peak power;
they utilize an energy storage system (capacitor
bank), which is trickle charged by the RPS.
34. 00-0642-34
Communications Requirements
• At a range of 100 AU, electromagnetic waves take 13.9
hours to travel from the spacecraft to Earth. Interactive
control of the spacecraft becomes nearly impossible at
these distances necessitating a highly autonomous
craft and one-way communications to Earth.
System Requirements Example System Properties
Requirement Value
Data rate 500 bps
9 Bit error rate 10-(encoding dependent)
Range (probe) 100 to 1000 AU
Range
High Earth orbit
(Earth terminal)
Probe tracking Sun tracker and star camera
Pointing
0.15 arcsec (0.72 μ rad)
accuracy
Electrical
power load
10 W (continuous)
Mass 10 kg
Lifetime >50 yr
Reliability 95%
Transmission
redundancy
Probe highly autonomous; transmissions
labeled with header and repeated X times
at predetermined intervals
Property Description
Probe light source Quantum cascade (QC) laser
Wavelength λ1 ~890 nm - near IR
Probe optical aperture 1 m (Gaussian beam angle is 1.13 μrad
Earth terminal aperture 4 m
Modulation
External binary phase shift modulation -
(probe to Earth)
BPSK
Modulation
(Earth to probe)
Amplitude modulation - binary
Earth terminal detection Coherent - homodyne
Probe terminal detection Incoherent - direct detection
35. 00-0642-35
Thermal Characteristics of Spherical
Thermal Shield
Spherical Shield with Secondary Shield
MLI on end of Spacecraft
3000
2500
2000
1500
1000
500
0
-500
-20 -15 -10 -5 0 5 10 15 20
Time (Hours)
Main Shield
Secondary Shield
S/C MLI on End
Spacecraft Radiator
Spacecraft
36. 00-0642-36
Perihelion Thermal Shield
• A carbon-carbon spherical shield protects the vehicle from
the Sun. The conical shape shown represents the available
volume for the probe and the propulsion model.
• A secondary shield further reduces the thermal soak back,
and a standard MLI blanket is used to protect the
spacecraft from the backside of the secondary shield.
• Although the shield temperature approaches 2700°C, the
spacecraft is protected.
• The thermal shield also must be incorporated into the
general mechanical design that must both accommodate
the propulsion system and allow for the spacecraft to work
under Orion-type or solar-thermal propulsion conditions.
37. 00-0642-37
Thermal Issues
• Ultra Low Power (ULP) electronics work best at low
temperatures. They use passive radiators to dump heat
generated by RPS and electronics. The goal is to operate
at 125–150K. No blankets are required.
38. 00-0642-38
Probe Thermal Design for Cruise
• The goal of the thermal design for the mirror and
its attached hardware is to operate between 75
and 12K.
• The thermal analysis assumes that the RPS
generates 100 W of thermal energy, with 15 W
going out as electrical power to the instruments.
The 85 W of heat are rejected at the RPS locally.
The ARPS is assumed to have a surface
temperature of 200°C.
39. 00-0642-39
Programmatics
• A 65-year technology development and flight program
can be done for ~$1000 M, exclusive of a DOE-cost
shared $800 M NTP development effort (if required).
• At an average cost of ~$15 M/year out of the NASA
Technology budget and with multiple commonalities
in technology requirements for other NASA missions,
such a development program:
• Makes sense as a science initiative and a
technology focus.
• Provides a low-cost means of reaching for the
stars.
40. 00-0642-40
Schedule
2000-2002 Advanced Technology Development study(ies)
2000-2002 Continued definition studies of the solar sail concept for IP at JPL
2002-2003 Update of OSS strategic plan with study for a "New Millennium”
-like mission
2003-2007 Focused technology development for small probe technologies
2004-2007 Development of sail demonstration mission
2004-2007 Development of Solar Probe mission (test for perihelion propulsion)
2006-2007 Hardware tests for radioisotope sail feasibility
2006-2007 Hardware tests for antimatter implementation in propulsion schemes
2006-2007 Monitor DoD STP effort and conduct NASA-specific hardware tests
2002-2007 Development of space-qualified nuclear thermal reactor
2007-2010 Focused technology development for an Interstellar Probe
2009-2012 Design and launch of first-generation solar-sail probe
2010 Test of Solar Probe performance in the perihelion pass of
October 2010
2012-2015 Design and launch second-generation probe 1000 AU goal in 50 years
2015-2065 Data return from out to 1000 AU
41. 00-0642-41
• Phase II funding authorized 17April 2000
• Subcontracts in process for mini-studies of:
• Advanced star tracker architectures
• Solar thermal propulsion to apply at perihelion
• Advanced LH2 storage for long-term, deep-space
propulsion
Current Status
42. 00-0642-42
Presentations and Papers
• “Low-Cost Interstellar Probe” talk presented 4 May 2000
at Fourth IAA International Conference on Low-Cost
Planetary Missions (Technical Session VI-Part 2: Outer
Planets Missions) Laurel, MD
• Corresponding MS submitted to proceedings as Paper
IAA-L-0608 (for publication in Acta Astronautica)
• Solicited presentation on Interstellar Probe concepts at
next meeting of the Sun-Earth Connections Advisory
Subcommittee (SECAS), NASA HQ, June 13–15, 2000.
• Abstract submitted to COSPAR Colloquium on The Outer
Heliosphere: New Frontiers, Potsdam, Germany, July
24–28, 2000.
43. 00-0642-43
This Step
• 50-kg probe is difficult but doable. Many new technologies
support this effort (ultra-low power CMOS, Explorer
Advanced Technology and PIDDP funding, optical and
wireless developments in commercial sector, etc.)
• Novel propulsion system is key. Three-stage Delta III
with 50-kg probe leaves only 250 kg for perihelion-burn
propulsion system.
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The Next Step
• What is needed next is another factor of 10 in speed, to
200 AU yr-1, at which the first targeted interstellar
crossing to Alpha Centauri will take ~1400 years,
the time that buildings have been maintained, e.g.,
Hagia Sophia in Istanbul (Constantinople) and the
Pantheon in Rome
Though not ideal, the stars would be within our reach.
